I. Field of the Invention
This invention relates generally to magnetron design, and is particularly concerned with an improved design for a cold or field emission cathode relativistic magnetron.
II. Description of Related Art
The conventional magnetron is a well-known and very efficient source of low frequency microwaves. Its operating principles have been known since at least 1921, and the first pulsed resonant cavity magnetron (3 GHz), built by the British in 1940, can be considered the germinal point of modern microwave radar. Today, magnetrons can be found in every home possessing a microwave oven.
A typical magnetron is a coaxial vacuum device consisting of an external cylindrical anode (the positive electrode, which attracts electrons) and an internal, coaxial cylindrical cathode (the negative electrode, which emits electrons). In many designs, resonator cavities of various shapes, such as rectangular, are cut into the anode block in a gear tooth pattern. During operation, a constant axial magnetic field fills the vacuum annulus, and an electric potential is placed between the anode and cathode. The number and shape of the resonator cavities, and the dimensions of the anode and cathode are arbitrary design features which determine the magnetron's frequency and operating characteristics.
Because of boundary conditions on electromagnetic fields at conducting surfaces, only certain field patterns sinusoidally oscillating in time at discrete frequencies (the "normal modes") will exist inside the magnetron cavity. These normal modes constitute a mathematically complete and orthogonal set, meaning any arbitrary electromagnetic field within the cavity can be decomposed into a sum of normal modes of the appropriate amplitudes and phases. The magnetron operation begins when an electric potential is applied between the two electrodes, initiating electron flow from cathode to anode. The axial magnetic field acts to insulate the electrodes by confining the electrons to the annular region inside the magnetron. The circular motion of electrons in the crossed electric and magnetic fields stimulates electromagnetic oscillations in the cavity, particularly when the velocity of the electrons matches the phase velocity of one of the normal mode components. As the wave gains energy, the fields back-react on the charge cloud to produce spatial bunching of the electrons, which in turn reinforces the growth of the wave. This bunching narrows the spectrum of preferentially activated modes. The preferred modes then gain energy at even faster rates and thus force even further bunching. The ideal magnetron design would quickly establish one dominant mode and one bunching pattern which stably and self-consistently reinforce each other. The conversion of beam energy to electromagnetic energy can be very efficient in magnetrons--as high as 70% in conventional devices.
Modern commercial magnetrons are typically of the hot (i.e., thermionic) cathode type and typically operate at voltages ranging from a few hundred volts to a few tens of kilovolts. Generally, electrons are produced in these devices by thermionic emission (i.e., heating) from the cathode. Currents of a few hundred amperes can be drawn in this way, and typical output power levels are tens to hundreds of kilowatts. The highest power achieved with this type of conventional magnetron was 7 MW.
In the past decade, the development of high voltage, kiloampere-level pulsed power drivers has led to a new class of experimental "relativistic magnetrons" which produce several orders of magnitude greater power. A magnetron of this type is described in U.S. Pat. No. 4,200,821 of Bekefi, et al. This experimental device used a field-emission cathode, in which the high electrostatic stresses draw large currents, and an anode resonator block having six identical resonator cavities, one of which is tapped for microwave extraction. Using 360 kV, 15 kA, and 0.8 T on a 3 GHz six-vane design, they reported 500-1000 MW in power over a 30 ns pulse. The magnetron has been tested at higher voltages to generate 3 GW of peak power. Further experiments have demonstrated a pulse length of 150 ns, but at a reduced power level of 100 MW. These achievements represent the present state of the art in high power relativistic magnetrons.
Other relativistic magnetrons based on different strategies have generally been less successful. Inverted magnetron designs have been tried, with the anode placed inside the field-emission cathode. This design reduced the current density required of the cathode, and also eliminated the undesirable azimuthal magnetic field resulting from the current injection. A 54-vane design reduced the resonant velocity of electrons. With an anode voltage of 580 kV, this design achieved 0.8 GW for 30 ns.
Deficiencies in the present high power microwave magnetron technology are evident, with the most serious being the inability to generate pulse lengths of a microsecond or greater. This is particularly critical for increasing the energy per pulse being produced. A magnetron producing 500 MW for 3 .mu.s would represent an order-of-magnitude increase in energy per pulse over the present experimental devices, and is greatly desired for practical applications.
The intrinsic limit to long pulses seems to be gap closure. Gap closure occurs when the formation of a plasma from electron bombardment of the anode interferes with the electromagnetic operation of the magnetron, either by providing a shorted current path, or by detuning the cavity. The pulse lasts for about the time it takes ions to cross the interaction region; this travel velocity is typically about 1 cm/.mu.s. In magnetrons with field-emission cathodes as described in U.S. Pat. No. 4,200,821, small, millimeter-size anode-cathode gaps are required to induce field-emission. This is counterproductive to long pulse lengths, since the transit time across a small gap is necessarily small. The problem is especially serious in relativistic magnetrons because megavolt potentials produce rapid acceleration of ions. Substantial anode damage is evidence of abundant ion generation. Nonetheless, for high power microwave generation field-emission cathodes are preferred over thermionic ones because of their ability to supply large currents. A long pulse, high power device would mark a major advancement of magnetron technology.
There are also practical problems with relativistic magnetrons. Anode erosion is severe because the large electron kinetic energy and the large currents produced in relativistic field-emission magnetrons rapidly degrade the surface quality of the anode, limiting the life of the device to a few hundred shots. The high voltages contribute to the gap closure problem. Although high power is achieved, conversion efficiencies seem to drop as relativistic energies are approached. Relativistic energies also require physically larger energy storage and magnetic field systems. Thus, there are a number of reasons why obtaining high power with nonrelativistic or moderately relativistic voltages would be a significant technological achievement.